In an effort to make vehicles more fuel efficient the automotive industry is making an all out effort to determine, and employ, acceptable ways in which to make their future models lighter.
At the present time vehicular shafts are normally made of tubular steel having a high flexural modulus--generally on the order of 30.times.10.sup.6 psi (2.1.times.10.sup.11 N/m.sup.2). The tubular steel drive shafts also have a high specific gravity--generally on the order of 7.7. As a result, the drive shaft itself is a relatively heavy item in an automobile.
It is well known that at certain rotational speeds a drive shaft becomes dynamically unstable and excessive vibrations are likely to develop. This phenomenon is a result of resonance, and the critical speed--i.e., the number of revolutions per second--at which this instability occurs is equal to the frequency of the natural vibration for the particular shaft. The critical speed for a drive shaft is generally proportional to the flexural modulus of the shaft and its moment of inertia, and, generally inversely proportional to its weight and length.
In order to maintain an acceptably high critical speed with tubular steel shafts, multiple, short sections are generally employed, each section being operatively connected to the successive section by a universal joint. The use of multiple universal joints operatively to interconnect the successive sections of the drive shaft further compounds the overall weight of the drive shaft assembly.
Fiber reinforced plastic shafts offer a distinctly suitable alternative to the steel, tubular shaft. Reinforcing materials such as glass, graphite and other fibers, or combinations of fibers, encapsulated in a thermosetting plastic matrix at a ratio of 50% to 70% fiber, by volume, produce fiber reinforced plastic materials having flexural moduli in the range of 5 to 25.times.10.sup.6 psi (3.5.times.10.sup.10 to 1.7.times.10.sup.11 N/m.sup.2) with a specific gravity in the range of 1.6 to 2.0, depending upon the particular materials and ratio employed.
An exemplary, fiber reinforced, hollow shaft, the method by which and the apparatus on which such a shaft can be made, are disclosed in U.S. Pat. No. 4,089,727, which patent is owned by our common assignee, Shakespeare Company.
The considerably lighter weight and larger moment of inertia of a fiber reinforced plastic drive shaft as compared to the weight of a tubular steel drive shaft taken in conjunction with only a moderately reduced flexural modulus allows a longer drive shaft to replace the multisection steel shafts at a comparable critical speed. Thus, not only is the weight of the shaft itself reduced but also, because the fiber reinforced shaft can be longer in comparison to a tubular steel drive shaft having the same critical speed, the weight of the complementary components can be reduced. For example, a number of the universal joints can be eliminated.
However, fiber reinforced members have heretofore not been considered satisfactory for automotive drive shafts for several reasons. First, no one fully appreciated the advantageous critical speed achievable with such a construction. Second, the satisfactory means of effecting an acceptable connection between the fiber reinforced drive shaft and the metallic yoke of a universal joint was unknown. Previous attempts demonstrated how difficult it is to effect a connection which can resist the forces imposed on the completed drive shaft assembly while maintaining the accurate alignment between the shaft and the structure by which the universal joint is secured thereto.
Utilization of fiber reinforced members for automotive drive shafts is therefore predicated upon the capability to form a bond between the fiber reinforced shaft and the yoke for accepting a universal joint which is able to endure the high torque and flexural forces, fatigue and temperature extremes from about -60.degree. F. to about 400.degree. F. (-50.degree. C. to 205.degree. C.) to which automotive drive shafts are subjected.
The bond between the fiber reinforced shaft and the yoke, or rather between the shaft and a sleeve to which a yoke is attached, has previously been effected by several methods. According to one method, a prepared and cured fiber reinforced shaft is either adhesively attached or mechanically lashed to the sleeve as by bolting or riveting. According to another method, the fiber reinforced shaft is fabricated by winding onto a mandrel containing the sleeve and thereafter the sleeve-shaft assembly is cured.
Fiber reinforced plastic drive shafts are generally comprised of several layers of fiber reinforced resin laminate. The innermost layer is comprised of fibers of glass, graphite or other materials, or combinations thereof, having a Young's modulus of at least about 10.times.10.sup.6 psi (6.9.times.10.sup.10 N/m.sup.2). In order to provide the hoop strength and stiffness necessary to prevent buckling and circular deformation of the shaft when under the torsional and flexural loads of operating conditions, the fibers of the innermost layers must be spirally wrapped that is, oriented relative to the longitudinal axis of the shaft at helix angles of less than 90.degree. but greater than about 80.degree..
Subsequent layers of such fibers are oriented with respect to the longitudinal axis of the shaft at helix angles which may vary from 0.degree. to about 65.degree., in order to provide the necessary properties for obtaining critical speed, torsional and flexural rigidity and strength to resist both torsional and flexural loads. Usually, the outermost layer of fibers is also wrapped spirally, that is, oriented at a helix angle of less than 90.degree. but greater than about 80.degree..
Each of the layers of fibers in the shaft have, conventionally, extended over the sleeve to form a bond between the shaft and sleeve. Included in this overlapping portion, is the spiral wrap having a helix angle within the range of about 80.degree. to just less than 90.degree..
Composite drive shafts having the above described construction have been found to be unsatisfactory when tested over the range of conditions incurred during automotive operation. One area in which drive shaft failure has occurred during these tests is in the sleeve-shaft bonding area.
We have discovered that drive shaft failure in the sleeve-shaft bonding area is promoted by the extension of the innermost spiral fiber wrap over the sleeve. Although the exact cause of this phenomenon is unknown, it is thought to result at least in part, from the varying thermal coefficients of expansion for the various materials comprising the completed drive shaft assembly.
The fibers utilized in composite drive shafts generally have lower longitudinal thermal coefficients of expansion than the metal sleeves contained therein, while the resins used generally have a higher coefficient. After the drive shaft assembly is heated, either during a curing step in the manufacture of the assembly or during operation in a vehicle, upon cooling the metal sleeve will contract away from the shaft portion. Although the resin would ordinarily tend to contract to a greater degree and thus adhere more firmly to the sleeve, it is prevented from doing so by the spirally wrapped fibers.
The fibers, having a lower thermal coefficient of expansion than the metal sleeve, will not contract upon cooling to as great a degree as will the metal sleeve. The hoop strength of the spiral wrap, needed to resist circular deformation of the shaft itself, in this instance prevents not only the fibers from conforming in any way to the contracted sleeve, but also prevents the resin structure which is bonded to the fibers from doing so. The adhesion between the shaft portions and sleeve is broken and drive shaft failure results.